The present invention relates to a new and improved method of, and apparatus for, determining a change of the flow state or condition of a flowable or fluent substance by means of ultrasonic energy or ultrasound.
In particular, the method for determining the change of such flow state contemplates transmitting ultrasonic or ultrasound waves of substantially constant frequency to a spatial region or specified volume of the substance, receiving the ultrasonic waves which are backscattered by the substance, converting the same into an amplitude- and phase-modulated primary signal corresponding to the backscattering and demodulating the primary signal in order to obtain a modulation signal which can be divided into a spectrum of signal components of different frequency and possessing pseudo periods which are defined as the time spacing between successive momentary values of the modulation signal and corresponding to one another. Further, the apparatus for determining the flow condition of a fluent or flowable substance is of the type comprising a signal generator for generating a reference signal of substantially constant frequency, an ultrasonic transmitter arranged in series with the signal generator for transmitting ultrasonic waves corresponding to the reference signal to a spatial region or specified volume of the substance. Further, there is provided an ultrasonic receiver for receiving ultrasonic or ultrasound waves backscattered at the substance and for converting the same into an amplitude- and phase-modulated primary signal corresponding to the backscattering. A demodulator serves for obtaining a modulation signal from the primary signal and an analyser serves for detecting signal components of the modulation signal.
Such type method and apparatus have already been disclosed in the publication of Shung, Sigelmann and Schmer in "IEEE Transactions of Biomedical Engineering" BME-22/4 (No. 6/1975) pages 334-337, and specifically in conjunction with the determination of the coagulation time of blood. Ultrasonic or ultrasound waves are backscattered or scattered both at the blood samples and also at the plasma samples, indicating that the ultrasonic waves are not exclusively scattered by the blood cells, rather also by other type of inhomogeneities of the sample liquid, for instance by micelles, density differences and the like. With the selected ultrasonic frequency every incoherent movement of the examined sample liquid, caused for instance by a turbulent state, convection or by Brownian movement, brings with it a modulation of the backscattered ultrasonic waves, and specifically a randomly controlled amplitude- and phase-modulation having a respective statistical distribution according to Rayleigh. From the aforementioned publication it is known to detect amplitude values of the backscattered ultrasonic waves as squared mean values, i.e., there is measured the intensity of the backscattered ultrasonic waves which prevails over a predetermined time duration. If there does not occur any incoherent movement in the sample liquid, then the measured intensity is essentially constant, its value corresponds to the backscattering of the ultrasonic waves by the static or stationary sample liquid and therefore does not amount to null. On the other hand, if incoherent movements arise in the sample liquid, then the intensity of the backscattered ultrasonic waves fluctuates about a mean value.
What is disadvantageous with the prior art measuring technique is that the point in time of transition from the incoherent to the coherent flow state or condition can only be determined inaccurately. Furthermore, this fact has been expressly mentioned in such aforementioned publication. During the formation of the intensity value as the squared mean value of the amplitude there occurs a suppression of high frequencies which thereafter cannot be rescinded. Consequently, the measuring technique is associated with inaccuracies or ambiguities in the time determination, and such constitutes an obstacle for automation of the measuring technique. In the aforementioned prior art publication there is not proposed any solution for overcoming such drawback.